Toners are broadly classified into pulverized toners, which are made from colored resin particles obtained by the pulverization method, and polymerized toners, which are made from colored resin particles obtained by the polymerization method.
In the pulverization method, a preliminarily-polymerized thermoplastic resin is used as a binder resin; a colorant, a charge control agent and other additives such as a release agent are added thereto; and the mixture is melted-kneaded, pulverized and then classified, thereby obtaining colored resin particles. A pulverized toner is produced by using the colored resin particles.
In the polymerization method, colored resin particles are obtained as follows: a polymerizable monomer composition containing a polymerizable monomer, a colorant and, if necessary, other additives such as a charge control agent and a release agent, is formed into minuscule droplets in an aqueous medium, followed by polymerization of the droplets, thereby obtaining colored resin particles. A polymerized toner is produced by using the colored resin particles.
In recent years, colorization of printed images has rapidly progressed in image forming devices using electrophotography, such as a copying machine and a printer. In color printing, high-resolution image printing is carried out, such as printing of pictures, so that there is a demand especially for high-resolution printing with high color reproducibility and there is also a need for high-quality color toners that can meet the demand. A spherical toner is suitable for color printing due to its high transferability and dot reproducibility. Since polymerization methods such as a suspension polymerization method, dispersion polymerization method and emulsion polymerization method can efficiently produce a spherical toner, a polymerized toner is suitable for color printing.
In recent years, environmental health regulations have been increasingly strengthened. In the field of image forming devices such as a copying machine and printer, impacts on human body and an odor of low-molecular-weight components remaining in a toner, which are components that will be volatilized when the toner is fixed by heat and include a polymerizable monomer, a decomposition product of a polymerization initiator, and other volatile organic substances, have become problems. When the low-molecular-weight components remain in large amounts in the toner, toner offset problems and toner filming problems on surfaces of members of image forming devices, such as a photosensitive member and developing blade, are likely to occur.
Therefore, methods for removing low-molecular-weight components from polymerized colored resin particles have been proposed. Due to the following reasons, it is more difficult to remove low-molecular-weight components in the polymerization method, than the case of the pulverization method. In the pulverization method, low-molecular-weight components can be removed by a heating treatment, etc., from a binder resin to be mixed with a colorant or other additive. In the polymerization method, colored resin particles are produced concurrently with polymerization; therefore, it is needed to remove low-molecular-weight components from colored resin particles in which a binder resin, which is obtained by polymerization of a polymerizable monomer, a colorant and other components co-exist. Low-molecular-weight components are easily absorbed into components other than the binder resin, such as a colorant, charge control agent and release agent. Therefore, it is more difficult to remove low-molecular-weight components from colored resin particles in the polymerization method, than the case of directly removing low-molecular-weight components from a binder resin in the pulverization method. When colored resin particles are heated for a long period of time or heated too much at high temperature for removal of low-molecular-weight components, the colored resin particles may aggregate or a colorant or other additives contained in the colored resin particles may deteriorate; therefore, the quality of a toner thus obtained is likely to be poor.
In Patent Literature 1, a method for producing a polymerized toner is disclosed, in which polymer particles are obtained by polymerizing a polymerizable monomer composition containing a colorant and a polymerizable monomer in an aqueous dispersion medium, and a stripping treatment is carried out on the polymer particles by pushing an inert gas and saturated water vapor at the same time into a dispersion containing the polymer particles.
In Patent Literature 2, a method for producing a toner for developing electrostatic images is disclosed, in which a polymerizable monomer composition containing a polymerizable monomer and a colorant is polymerized in an aqueous dispersion medium in the presence of a specific polymerization initiator to form colored resin particles, and volatile substances remaining in the colored resin particles are removed by stripping. In claims 1 and 4 of the literature, the content of styrene (a polymerizable monomer) and the content of an ether component (a decomposition product of a polymerization initiator) are mentioned.
As a result of researches made by the inventor of the present invention, it has been found that the method disclosed in Patent Literature 1 is problematic in that when stripping is carried out too much, an odor generated from the toner upon fixing, can be reduced; however, insufficient low-temperature fixability is obtained. In Patent Literature 2, experimental results on fixability are not disclosed at all.